1. Field of the Invention
The present invention relates to an in-line type color cathode ray tube (CRT), and more particularly, to a convergence magnet assembly installed in the neck of a CRT for adjusting the static convergence of an electron beam of the CRT.
2. Description of the Related Art
Generally, in a color CRT having in-line type electron guns, the axes of three electron guns are arranged in the same plane including the tubular axis of the CRT such that the paths of two outside electron beams emanating from the electron guns located on both sides of the central electron gun will be symmetrical with respect to the path of the electron beam emanating from the central electron gun. Thus, it is advantageous to construct the electron gun assembly of such a CRT to cause the electron beams emanating from the respective guns but not deflected by deflecting means to converge at the center of a fluorescent screen located in front to the electron gun assembly so as to produce clean pictures.
However, it is difficult to cause picture tubes to satisfy such ideal conditions due to manufacturing errors in components parts of the tube or permissible dimensional allowances of the component parts.
For this reason, the color CRTs generally have slight variations in the path characteristics of the electron beams. It is known that the paths of electron beams are modified to compensate for such variations in electron beam path characteristics. The paths of the electron beams can be modified by an adjustable magnetic field, which has been used to provide a required color purity adjustment as well as static convergence adjustment.
That is, as shown in FIG. 1, the color purity has been adjusted by relatively rotating two annular shaped two-pole permanent magnets 11 and 11' mounted parallel to each other on a neck portion 101 of a CRT 100, surrounding the paths of the electron beams. Also, a static convergence adjustment has been accomplished by providing two annular shaped four-pole permanent magnets 12 and 12' and two annular shaped six-pole permanent magnets 13 and 13' mounted parallel to each other on the neck portion 101 of the CRT surrounding the paths of the electron beams. When the magnet assembly 10 is aligned, the intensity of the magnetic field of the magnet assembly is maximized. On the other hand, when two permanent magnets are rotated relative to each other over a predetermined angle, e.g., 180.degree., 90.degree. or 60.degree., the intensity of the magnetic field of the magnet assembly is minimized.
Where the CRT is ideally manufactured according to a design rule, a static convergence error is not generated or few errors are generated. Thus, it is not necessary to provide the magnet assembly. The static convergence is dislocated due to the minimum magnetic field of the magnet assembly mounted on the neck portion of the CRT. Accordingly, the convergence magnet assembly should be manufactured such that shifting of an electron beam is minimized. In the magnet assembly having a minimum beam shift, as shown in FIG. 2, if the distance OCV between the outer two electron beams, among three electron beams in an in-line arrangement, and the distance CCV from the midpoint of the distance OCV and the central beam deviate from the range A of the minimum shift amount, it is difficult to adjust static convergence.